Push the needle (ideal for the job long, pointed but nice rounded tip) under the grip, spray a little WD40 in and work the needle ground the grip, and then slide or peel off.
Worked a treat when I changed to heated grips a couple of months ago. Still have the original grips intact if I need them again.
If you remove the switch/throttle assembly
from the bars it is usually easy to push the
wide end of the grip and slide it away from
you and off the bars.
I prefer not to use WD40 or similar but get
the new grips and bars really clean before
fitting otherwise they can slip when riding.
Easiest and quickest way is to get a compressor , they usually have a normal nozzle attachment and stick it under the grips and blow , mine came off in seconds with no residues at all !
I use WD40 and an old stainless steel car wiper blade stiffener, round the tip on a grinding wheel first, similar mode to the knitting needle but easier to insert.
Give the bars a through clean with methylated spirits to remove the WD40 residue and any other crud you don’t want under your grips.